Regional Reservoir Compartmentalization within offlapping, Top-Truncated, Mixed-influenced Deltas, Wall Creek Member, Frontier Formation, Powder River Basin, Wyoming

Sadeque, Junaid¹, Janok P. Bhattacharya² (1) Chevron ETC, Houston, TX (2) University of Houston, Houston, TX

This study utilizes a dataset of over 2000 well logs and 40 cores linked to the adjacent outcrops exposed along the western margin of the Powder River basin, Wyoming in order to document the stratal inter-relationships, sandbody geometries and reservoir heterogeneity of the Upper Turonian Wall Creek Member of the Frontier Formation. We show that the Wall Creek Member, previously interpreted as an offshore bar or shelf ridge, is better interpreted as a delta system formed during a lowstand, individual lobes of which have been top-truncated by marine ravinement during high frequency episodes of transgression. Basinwide correlations and mapping reveal consecutive stacks of lensiod shaped, offlapping deltaic parasequences that prograde progressively basinward towards the south-southeast. Sub-regionally, the Wall Creek compartmentalizes into a two tier system of parasequence sets, separated by a major flooding surface. Individual parasequences within each set are locally farther compartmentalized into bedsets. Differential accommodation caused by proto-Laramide tectonics probably enhanced amalgamation and top-truncation of these parasequences towards the north.

Internal facies architecture of preserved prodelta and delta front deposits show varied influence by river, tide and wave processes. Sandbody isolith maps show strike-elongate to lobate and dip-elongate paleogeomorphology, which also reflect different degrees of reworking by waves and tides over time.